Wetwood, or water pocket, has a water-soaked appearance that occurs in the wood of living trees, including many hardwood and softwood species (Libra, 1999). Wetwood is caused by anaerobic bacteria that enter wood through wounds and lesions in trunks and roots of young trees. The affected wood is dead and changes to yellowish green or without any distinctive colour but can be recognized by its sour odour. Compared with normal sapwood, wetwood has less extractible carbohydrate content, such as sugars, but higher level of mineral and organic compounds. The zone of wetwood has a pH of 1-2 units higher than unaffected areas, and contains gas under pressure.
Cellulose, hemicellulose and lignin are the main components of wood cell walls, and the chemical structure and interrelationships of these components in wood cells are well known (Zabel and Morrell 1992). In addition to these main structural compounds, pectin is another polymer commonly located in the middle lamella and primary cell wall of trees, and acts as a cementing substance there (Schink et al. 1981a). Several anaerobic fermentative bacteria produce pectinolytic enzymes that destruct vessel and ray pit membranes of wood (Schink et al. 1981b). The reproduction and metabolites of these bacteria form a foetid liquid in wood, which results in a high moisture content of the wetwood. Since moisture content of wetwood is much higher than the normal wood, it usually requires relatively long periods for adequate drying (Schneider and Zhou 1989). The slow drying of the wetwood pocket compared to the surrounding wood, possibly due to blockage of pathways by bacterial mucilage, creates steep moisture gradients creating greater internal stresses within the lumber. Furthermore, degradation of pectic substances of the middle lamella causes the weakness of chemical bonds between wood cells. Consequently, weak bonding causes a high risk for developing checks, splits, crook, bow and twist of lumber in kiln drying (Ward and Pong 1980, Verkasalo et al. 1993). The lower permeability of wetwood compared to normal wood also affects the wood's treatability with preservatives.
Wetwood causes serious problems for lumber drying and utilization. The economic losses resulting from this defect are enormous. If wetwood problem can be solved, the value of lumber will be increased and the benefit for sawmills will be significant. Based on a conservative estimate, if lumber grade recovery can be increased by 5% and drying times can be reduced by 15% with a defined method (Linares-Hernandez and Wengert 1997), a sawmill producing 100 MMfbm per year with 20% lumber containing wetwood would save over $500,000 per year.
Many studies have been conducted on wetwood using various physical, chemical, or mechanical methods, but the problem has yet to be solved.